This invention concerns a method of forming insulating films, and in particular a method of forming high quality insulating films of very small thickness.
In silicon integrated circuits, and in particular in MOS (Metal Oxide Semiconductor) integrated circuits formed by modern techniques, very thin oxide films are used as gate insulators. For sub-micron MOS devices which have gate lengths of 1.0 .mu.m or less, the oxide film may for example be no more than 100 Angstroms thick, this reduction in thickness permitting substantial gains to be made.
These oxide films may be formed by the following method which is described in "MOSLSI Seizo Gijutsu" (The Manufacture of MOSLSI), by Takashi Tokuyama and Tetsuichi Hashimoto, published by Nikkei McGraw Hill Co., p. 64 (1985).
According to the method disclosed in this reference, a substrate that has been cleaned is placed in a quartz tube heated to 800.degree.-1200.degree. C. by an electric furnace, and an oxidizing gas is then introduced into the tube to form an oxidizing film. This oxidizing gas may for example be dry oxygen gas, or a mixture of oxygen gas and hydrogen gas, or a hydrochloric acid spray mixed with oxygen gas. As shown by the dotted line I in FIG. 4, which is a plot of oxidation time (sec) on the horizontal axis versus oxide film thickness (Angstroms) on the vertical axis, there is a definite relation between these two variables. It is therefore possible to form an oxide film of a desired uniform thickness on the substrate by allowing it to grow for a suitable time at a suitable temperature.
In the above method of forming oxide films, however, the film is grown continuously without interruption, and it was therefore difficult to control the thickness of very thin films in the region of 100 Angstroms or less. An attempt was made to solve this problem by reducing the oxidation temperature below 800.degree. C. to decrease the rate of oxidation, or by diluting the oxygen with nitrogen.
At lower oxidation temperatures, however, the silicon/silicon dioxide interface is rougher. In the diluted oxidation method, on the other hand, nitrogen segregates at the silicon/silicon dioxide interface, and the interface state density is therefore increased. Both of these methods therefore failed to offer any improvement of properties such as the endurance against dielectric breakdown of thin oxide films.
Furthermore, the oxide films obtained at low temperatures or by diluted oxidation are generally not very dense. At the silicon/silicon dioxide interface, for example, there are a large number of unterminated bonds of silicon atoms or strained Si-O-Si bonds. As a result, the interface state therefore showed a tendency to increase. These phenomena gave rise to various problems when the oxide films were used as gate dielectric in MOSFETs. In MOSFETs with a gate length of 1.0 .mu.m or less, for example, if hot electrons produced in the channel region are injected into the oxide film, the electrons were trapped by unterminated bonds of silicon atoms or strained Si-O-Si bonds, and a new interface trap state was generated. This led to an instability of the threshold voltage, or a degradation of the transconductance, in MOSFETs.
To solve these problems, the authors of the present invention carried out various studies and experiments. They then found that if the thin film was not grown continuously as in the prior art, but grown in several stages instead, there was no risk of surface roughness or of causing an increase in interface state density.
They also found that if the insulating film that was grown in several stages in this way, was crystallized prior to or at the same time as the next layer to be grown on top of the film, a high regularity compliance, high quality crystalline insulating film is obtained.